1. Field of the Invention
The present invention relates generally to die casting. More specifically, the invention relates to the design of die casting systems.
2. Description of the Related Art
Casting is a manufacturing process by which liquid material is introduced into a mold, which contains a hollow cavity of a desired shape and then allowed to solidify. Die casting is the injection of molten metal under high pressure into a steel mold, interchangeably referred to as a die, for the purposes of rapid manufacturing at rapid production rates. The molten metal is most often a non-ferrous alloy, which is used because the best performance for die-cast products is gained through a blend of materials. Some typical alloys that are used for die casting are aluminum alloys, magnesium alloys and zinc alloys, which can contain other elements such as silicone.
Two methods can be used to inject molten metal into a die; cold chamber and hot chamber. A schematic illustration of a typical cold chamber die casting machine 600 is shown in FIG. 1A. The die casting machine 600 comprises a mold 602 made of tool steel in at least two die halves 604, 606 that together define a part cavity 608. The cover half 604 is held by a fixed machine platen 605, and the ejector half 606 is held by a moving machine platen 607 so that the ejector half 606 can move back and forth to open and close the mold 602. Molds 602 often also have moveable slides, cores, or other sections to produce holes, threads, and other desired shapes in the casting. Molds 602 are alternately referred to as dies or tools.
The die casting machine 600 further includes a pressure chamber 610 through which molten metal from a supply 612 is delivered or injected into the mold 602 using a plunger 614. One or more shot sleeves 616 in the cover half 604 allow molten metal to enter the die and fill the part cavity 608. When the pressure chamber 610 is filled with molten metal, the plunger 614 starts traveling forward and builds up pressure, thereby forcing the metal to flow though the shot sleeve 616 to the part cavity 608. After the metal has solidified, the plunger 614 returns to its initial position, and the ejector half 606 of the die opens for the part or casting to be removed from the mold 602. Ejector pins 617 are used to push the casting out of the ejector half 606 of the mold 602. This process is referred to as a single casting cycle. Multiple casting cycles can be completed during a die casting operation.
A schematic illustration of a typical cold chamber die casting mold 602 is shown in FIG. 1B. The die casting mold 602 comprises a biscuit 618, which is the remaining material in the shot sleeve 616 after the shot is complete. One or more runners 620 connect the shot sleeve 616 to corresponding gates 622 through which molten metal enters the part cavity 608. One or more overflows 624 are connected to the part cavity 608 to receive the first molten metal that enters the part cavity 608 because it is usually contaminated with petroliates from the die spray applied to the mold 602 in previous casting operations.
Cooling lines 626 run throughout the mold 602, through which coolant, such as water or oil, flows to aid in the removal of heat from the mold 602. There are a number of individual cooling lines 626 that are responsible for cooling different parts of the casting or shot. The number of cooling lines 626 in a mold varies according to the size of the mold. For example, a small mold may have fifteen cooling lines, while a large mold may have over a hundred cooling lines. The cooling lines 626 are all in communication with a coolant flow system (not shown), from which coolant is delivered to the cooling lines, and to which coolant returns after it flow through the cooling lines. Many coolant flow systems for dies are part of a plant-wide water system. Other coolant flow systems are “closed-loop” systems, in which coolant is only cycled through the coolant flow system.
The casting can be divided into multiple heat flow zones that are cooled by one or more cooling lines 626. The heat flow zones are generally indicated by the dotted boxes on FIG. 1B. The heat flow zones of the casting comprise the biscuit (Zone 0), the main runner (Zone I), the gate runner (Zone II), the gate side of the casting, also referred to as the gate side of the part (Zone III), the overflow side of the casting, also known as the gate side of the part (Zone IV), and the overflow (Zone V). The biscuit (Zone 0) generally corresponds to the biscuit 618. The main runner (Zone I) corresponds to the portion of the runners 620 that are closest to the biscuit 618. The gate runner (Zone II) corresponds to the portion of the runners 620 that is closest to the part cavity 608. The gate side of the casting (Zone III) is the casting half nearest to the gates 622. The overflow side of the casting (Zone IV) is the casting half furthest away from the gates 622. The overflow (Zone V) generally corresponds to the overflows 624.
There are primarily three critical die-casting process control requirements. The first requirement relates to the timing and function of the die casting machine. The timing of the opening and closing of the mold must be closely managed during the process to sequence operations such as injecting metal into the part, dealing with moving slides, making any intricate details in the casting, and extracting the part. The timing of these and other operations can be controlled to optimize the production rate and quality of the castings.
The second requirement relates to the injection processes at the shot end of the die casting machine. The injection processes, both from the standpoint of hardware and software, have been developed over time to optimize the control of injecting the liquid metal into the mold. Injection speed, injection pressure, and flow rate are all involved in the control of the injection process and can be taken into account during the design of the die casting process. Technologies have been developed to address the first two requirements in terms of machine design and shot end design to manage the first two problems that die casters have dealt with.
The third requirement relates to the thermal design, monitoring and control of the die casting process, including temperature detection and the removal of heat from the mold. Thermal design encompasses designing the cooling system of a die casting machine, which includes determining the number of cooling lines, the placement of each cooling line relative to the part cavity, the depth of each cooling line relative to the die surface, using the appropriate size, i.e. diameter, of cooling line, and determining the appropriate flow rate of each cooling line. Misplacement of cooling lines in a die casting mold results in a mold that does not cool properly, which can lead to subpar casting performance and poor quality castings. Thermal monitoring refers to monitoring temperature and heat during the actual use of the die. Thermal control encompasses taking the information gathered from thermal monitoring and responding to that information, with respect to the intended thermal design.
Thermal design has historically been haphazard in the engineering of die casting processes. This is partly because the mathematics involved in thermally designing a die can be complex. The typical standard for thermal design in the die casting industry is based upon experimental knowledge of the die casting process. Often a designer works with a toolmaker to roughly estimate the appropriate location of cooling lines. A computerized thermal simulation can then be used to evaluate a potential design. The results of the simulation are reviewed, and the design is modified based on the results. An iterative process of estimating the position of cooling lines, running a computerized thermal simulation, reviewing the results and then modifying the position of the cooling lines based upon the results is required to achieve a suitable die design. After multiple iterations, a fairly good design can be achieved, but it might not be the optimum process for all die casting systems. Also, designing a die casting system using the iterative process is slow because of the potentially high number of iterations that must be completed until a suitable design is found.
Another problem is that most simulation software was founded in gravity casting techniques. Gravity casting uses sand molds that operate like insulators, so little heat is lost during a casting operation. In die casting, as the liquid metal enters into the mold, a significant amount of heat is lost during filling, which is not taken into account by the simulation software.
The use of plant-wide water systems as a source of coolant also effects the thermal aspects of a die casting process. Such water systems have significant variation in temperature of the water, which is difficult to account for during thermal design of a die casting machine and is difficult to control during operation. There are also chemistry concerns when using a plant-wide water system, because contaminants such as calcium and lime in the water can cause a film to develop on the cooling lines, which restricts heat transfer between the mold and the coolant.
Die life is a function of thermal fatigue in the mold. Thermal fatigue is brought on by the temperature variation that the mold steel must withstand during production. The level of thermal fatigue is driven by the accuracy of the location of cooling lines because if there is inaccuracy in position of the cooling line, the surface temperature of the mold can be an undesirable level. For example, when using an aluminum alloy for casting, the die surface temperature at injection is typically near 600° F. If the cooling line is placed too far away from the die surface, the die surface temperature may be allowed to rise to as high as 800° F. Since the coolant is not removing the appropriate amount of heat, an external spray must be used. External sprays, which contain petroliate, are normally only used during die release to allow the casted part to be more easily extracted from the mold, but because of the high die surface temperature caused by misplacement of the cooling lines, the external spray, which may be further diluted with water, is used to cool the die during the casting process.
In such cases where the die surface temperature is unusually high and the external temperature of the die is low from the use of an external spray, a phenomenon called “heat checking” occurs, which is the effect of exceeding the fatigue limit of the material the mold is made from. All molds have minute cracks in the mold surface, and liquid metal enters these cracks during a casting operation. Heat checking thermally insulates the die and reduces the potential heat removal from the die. Eventually the mold can crack, and pieces of the mold may stick to the part, which can cause a number of problems such as poor part quality or part failure.
Another problem with current die casting thermal design is that little emphasis has been given to the location of shrinkage porosity defects in the product and to controlling the dimensional accuracy and precision in relation to gas porosity defects. When the casting material goes from liquid to solid, its density increases, which causes a volumetric void to form near the last area to solidify within the casting. Improper thermal design of the mold can cause a volumetric void to form in an undesired location in the casting, potentially forming a defect in the final product.
With regard to dimensional accuracy and precision in relation to gas porosity defects, the die casting process has long been considered a net shape process, but not an accurate one. The reason behind the poor dimensional accuracy and precision is that since the injection temperature of the liquid metal varies in different sections of the casting, the casting is often ejected at an inconsistent temperature. The shrinkage that the casting undergoes will be inconsistent as well since the entire casting has to cool down to ambient temperature. For example, if one section of the casting is at a temperature of 800° F. at ejection, and another section of the casting is at 300° F. at ejection, the section at 800° F. will undergo more shrinkage than the section at 300° F. This inconsistent shrinkage can create distortion and dimensional inaccuracy in the casting, potentially necessitating additional machining operations on the casting to achieve reasonable dimensional control.
Another problem associated with poor thermal design occurs during the process of ejecting the casting from the mold. If there is a “hot spot” in the die, i.e. a portion of the die that retains more heat than the rest of the die, ejection is delayed because that area of the casting must cool longer than the rest of the casting, which means that the remainder of the casting will be cooler than it needs to be for ejection. When the casting cools too long within the mold, it can contract around details in the die and may increase the force needed to eject the casting, which can cause distortion or cracking of the casting. Waiting for the hot spot to cool also results in longer cycle times. Hot spots may also cause soldering to occur, which is when the temperature of a portion of the die is so high that the die spray burns off and the casting sticks inside the part cavity. The casting may still be ejected, but some of the casting material may stick to the die and oxidize.